Orthographic camera mirrored in viewport?
This is because the Object mode Mirror tool works by scaling -1. (the same thing will happen if you manually scale the camera by -1.) You will also notice that in addition to the viewport being mirrored, Object Normals appear reversed.

from the wiki about Edit mode mirror:


  The mirror tool in Edit mode is similar to Mirroring in Object mode.
  It is exactly equivalent to scaling by -1 vertices, edges or faces
  around one chosen pivot point and in the direction of one chosen axis,
  only it is faster/handier. 


note that using an Orthographic or Perspective camera does not change this behavior. It is the same on both.

(as already mentioned by Polosson),
The solution is to Rotate the camera instead of mirroring it.

Also as already mentioned by Polosson, one way to do this would be to


Select the building, and press Shift+S > Cursor to selection
Press . to change the Pivot point to Cursor
Select the Camera and rotate around the Z axis by 180 degrees. (R+Z+180)